On June 15,
1991, Mount Pinatubo in the Philippines exploded in the second largest volcanic
eruption on Earth this century. This eruption deposited more than 1 cubic mile
(5 cubic kilometers) of volcanic ash and rock fragments on the volcano's slopes.
Within hours, heavy rains began to wash this material down into the surrounding
lowlands in giant, fast-moving mudflows called lahars. In the next four rainy
seasons, lahars carried about half of the deposits off the volcano, causing even
more destruction in the lowlands than the eruption itself.

Background

Before April 1991, when steam explosions blasted out three small craters on
the north flank of Mount Pinatubo on the island of Luzon in the Philippines, the
volcano had been dormant for 500 years, allowing lush tropical vegetation to
cover its slopes. Eventually, the population of the region grew to nearly
1,000,000 people, as towns, cities, and hundreds of villages, as well as the
largest U.S. military base in the Philippines (Clark Air Base), were built on
the broad gentle slopes surrounding the volcano.

When Pinatubo reawakened in April 1991, scientists from the Philippine
Institute of Volcanology and Seismology (PHIVOLCS) intensified their monitoring
of the volcano; they were soon joined by scientists from the U.S. Geological
Survey (USGS) and other organizations. While evaluating the threat from the
volcano, these scientists discovered that the gentle slopes surrounding Pinatubo
had largely been created by countless giant mudflows of volcanic debris (lahars)
from the powerful ancient eruptions that built the volcano. They also concluded
that lahars would again rush down Pinatubo's slopes in its next major eruption,
threatening the communities that were built while it slumbered.

In August 1991, 2 months after Mount Pinatubo,
Philippines, exploded in a cataclysmic volcanic eruption, this house (see
roof) along the Sacobia-Bamban River 15 miles from the volcano was buried
by a fast-moving mudflow of volcanic ash and debris (lahar) caused by
heavy rains. Inset photo shows the house before it was buried. (Photos by
Raymundo S. Punongbayan, Philippine Institute of Volcanology and
Seismology.)

Nature and Causes of Lahars

Explosive eruptions can deposit huge amounts ash and other volcanic debris on
a volcano's slopes. Lahars form when water from intense rainfall, melting snow
and ice, or the sudden failure of a natural dam, mixes with this loose volcanic
material, creating mudflows that can be particularly dangerous and destructive.
Although lahars contain at least 40% (by weight) volcanic ash and rock
fragmentsmaking them dense and viscous like wet
concretethey actually flow faster than clear-water streams. These
mudflows can rush down the flanks of a volcano at speeds as great as 40 miles
(65 kilometers) per hour and can travel more than 50 miles (80 kilometers).
Lahars that contain the most debris (as much as 90%) move the fastest and are
the most destructive.

The village of Lourdes, built on lowlands 15 miles
northeast of Mount Pinatubo, was inundated by giant, fast-moving mudflows
of volcanic debris (lahars) in the months following the volcano's
cataclysmic June 1991 eruption. Since that eruption, lahars have destroyed
the homes of more than 100,000 people in the area surrounding
Pinatubo.

Conditions that Create Lahars at Pinatubo

Annual rainfall at Mount Pinatubo ranges from 80 inches (2,000 millimeters)
on the volcano's northeast flank to more than 160 inches (4,000 millimeters) on
its summit and southwest flank. Nearly all of this rain falls during the time of
the southwest monsoon (June to September) and the overlapping typhoon season
(June to October). Rainfall during typhoons can be as intense as 2 inches (50
millimeters) per hour. Since the volcano's cataclysmic June 1991 eruption, the
maximum rainfall at Pinatubo in one 24-hour period was more than 30 inches (780
millimeters)as much rain as many parts of the United States receive
in a whole year!

Ash and other deposits from Mount Pinatubo's 1991 eruption destroyed or
buried much of the lush vegetation that had covered the volcano. When rainfall
on Pinatubo exceeded about 0.5 inches (12 millimeters) in 30 minutes, rapid
runoff down the still bare slopes quickly grew into torrents that became lahars
by eroding these deposits. Fortunately, vegetation is beginning to grow on the
1991 deposits, and new stream channels are becoming established, reducing the
runoff and erosion that create lahars.

Lahars at Pinatubo Since 1991

The volume of Pinatubo's lahars staggers the mind. In the first few years
following the cataclysmic 1991 eruption, they have deposited more than 0.7 cubic
miles (3 cubic kilometers; equivalent to 300 million dump-truck loads) of debris
on the lowlands surrounding the volcano, burying hundreds of square miles of
land. During heavy rains, lahars at Pinatubo can transport and deposit tens of
millions of cubic yards of mud in a single day.

When the largest and fastest lahars reach the lowlands surrounding Pinatubo,
they have speeds of more than 20 miles (32 kilometers) per hour, are as much as
30 feet (10 meters) thick and 300 feet (100 meters) wide, and can transport more
than 35,000 cubic feet (1,000 cubic meters) of debris and mud per second. Within
minutes, such powerful lahars can erode many feet into the banks or floor of a
channel. They can carry large boulders, cars, or even bridges and can destroy or
bury almost anything in their paths. When lahars reach low-lying areas, they
spread out, slow down, and deposit their vast loads of debris over many square
miles.

One particularly dangerous type of lahar has occurred at Mount Pinatubo as a
result of the damming of stream channels and tributary valleys by the 1991 ash
deposits or by subsequent lahars, creating temporary lakes. After a few weeks or
months, the lakes overtop and quickly erode through their dams, releasing large
volumes of water that generate major lahars. Because these large and powerful
mudflows can occur without warning, even when there is no rain, they are among
the most dangerous of Pinatubo's lahars. One lake on the Pasig-Potrero River, on
the volcano's east flank, has formed and broken out three times (1991, 1992, and
1994), creating giant lahars that have killed dozens of people.

Mitigation Efforts and Continuing Hazards

When Pinatubo erupted in 1991, stream channels were small and were quickly
filled by lahars deposits. Civil engineers raced to trap the sediment behind
small dams and to confine it between levees, but most of these measures were too
small to contain the lahars. Near the base of the volcano, lahars eroded whole
city blocks, and in the lowlands they inundated and buried entire towns.

Since the 1991 eruption, lahars from Pinatubo have destroyed the homes of
more than 100,000 people. With most old stream channels filled, lahars can now
spread widely on the gentle slopes and lowlands surrounding Mount Pinatubo,
threatening successors to the early dams and levees. Fortunately, by 1995, the
volume of material carried by Pinatubo's lahars had already decreased to less
than a quarter of that carried in 1991, and new towns had been built on high
ground for many of those displaced or threatened by lahars. Nonetheless, 100,000
people remain at risk from the volcano's lahars.

Within hours of Mount Pinatubo's explosive June 15, 1991,
eruption, heavy rains began to wash the ash and debris deposited by this
explosion down into the surrounding lowlands in giant, fast-moving
mudflows called lahars. By 1993, lahars had already caused more
devastation in the lowlands than the eruption itself. Fortunately, a
system to monitor and warn of lahars was rapidly established in the days
following the eruption by the Philippine Institute of Volcanology and
Seismology, U.S. Geological Survey, and other organizations. This system
has saved hundreds of lives by enabling warnings to be sounded for most
but not all major lahars at Pinatubo. U.S. bases have reverted to
Philippine control since 1991. Subic Bay and Cubi Point are naval
stations. 1, O'Donnell River; 2, Sacobia-Bamban River; 3, Pampanga River;
4, Pasig-Potrero River; 5, Marella River.

Lahar Warnings

Once the potential for lahars at Mount Pinatubo was recognized, scientists
quickly prepared a preliminary hazard map. Since mid-June 1991, when violent
eruptions triggered the volcano's first lahars in 500 years, a system to monitor
and warn of lahars has been in operation. Radio-telemetered rain gauges provide
data on rainfall in lahar source regions, acoustic flow monitors on stream banks
detect ground vibration as lahars pass, and manned watchpoints further confirm
that lahars are rushing down Pinatubo's slopes. This system has enabled warnings
to be sounded for most but not all major lahars at Pinatubo, saving hundreds of
lives.

COOPERATING
ORGANIZATIONS
Armed Forces of the Philippines
National Disaster Coordinating Council, Philippines
Philippine Institute of Volcanology and Seismology
U.S. Agency for International Development
United States Air Force
United States Navy